Ion implantation method has been used for placing impurity, or doping, ions in a semiconductor material such as in a silicon substrate at precisely controlled depths and with accurate control of dopant ion concentration. One of the major benefits of the method is its capability to precisely place ions at preselected locations and at predetermined dosage. It is a very reproducible process that enables a high level of dopant uniformity. For instance, a typical variation of less than 1% can be obtained across a wafer.
An ion implanter operates by providing an ion source wherein collisions of electrons and neutral atoms result in a large number of various ions being produced. The ions required for doping are then selected out by an analyzing magnet and sent through an acceleration tube. The accelerated ions are then bombarded directly onto the portion of a silicon wafer where doping is required. The bombardment of the ion beam is usually conducted by scanning the beam or by rotating the wafer in order to achieve uniformity. A heavy layer of silicon dioxide or a heavy coating of a positive photoresist image is used as the implantation mask. The depth of the dopant ions implanted can be determined by the energy possessed by the dopant ions, which is normally adjustable by changing the acceleration chamber voltage. The dosage level of the implantation, i.e., the number of dopant ions that enters into the wafer, is determined by monitoring the number of ions passing through a detector. As a result, a precise control of the junction depth planted in a silicon substrate can be achieved by adjusting the implantation energy, while a precise control of the dopant concentration can be achieved by adjusting the dosage level.
When a silicon substrate is implanted, a high energy ion beam enters a perfect crystal structure below the wafer surface to knock atoms out of the crystal lattice and changes the top layer of a few thousand angstroms thickness to an amorphous structure. The physical change from a crystalline structure to an amorphous structure radically changes the electrical properties of the silicon material and causing damages to an otherwise perfect crystal structure. Generally, the higher the implantation dosage used, the higher the degree of damage is made to the crystal lattice of the silicon material. While the surface layer of the crystal lattice in the silicon is temporarily damaged by the implanted ions, the silicon atoms in the lattice including those which are knocked out of the lattice sites have certain degree of self-repairing capability. This self-repairing capability is normally triggered by thermal energy and the tendency that a crystal structure normally resumes its original condition after a disturbance in its structure has occurred.
An important aspect in the application of the ion implantation technology is the capability of measuring the degree of implantation in the silicon material. In other words, the capability of measuring the disturbance or damage made by the impurity or dopant ions to the perfect crystal structure of silicon. One of the methods capable of measuring the degree of implantation is a TP-400 XP system made by the Therma-Wave Company. The system is utilized for determining the degree of damage in the crystal lattice of a silicon wafer after an ion implantation process. The system is used in semiconductor fabrication processes to first determine the degree of damage in a crystal structure, and then using such data to determine the various implantation parameters such as the implantation energy, the type of impurity ions to be implanted, and the dosage of the ions implanted. An optimal implantation process can then be carried out by using these selected processing parameters based on the test result obtained from a TP-400 XP system. After such an optimum implantation process is determined, the processing parameters can be used for volume. production of a semiconductor device from silicon wafers.
A schematic diagram of the TP-400 XP system 10 is shown in FIG. 1. An argon ion pump laser 12 having an intensity modulation of 1 MHz emits a laser beam 14 through a beam splitter 16 and a condenser lens 18 to irradiate on a silicon wafer 22. The laser beam 14 is condensed into a focused beam 26 having a width of approximately 1 micron at the point in contact with the top surface 24 of the silicon wafer 22. The plasma generated by the argon ion pump laser reacts with the silicon crystal structure damaged by the ion implantation process in the surface layer of the wafer and then reflects from the surface 24. The reflected wave 32 is detected by another helium neon probe laser 28. Assuming the intensity of the incident wave 14 is R and the intensity change in the reflected wave 32 is .DELTA.R, then detector 34 detects a thermal wave (TW) signal which represents .DELTA.R/R. A second beam splitter is used in the path of the detector 34.
FIG. 2 shows a graph illustrating the dependency of the thermal wave signal on the implantation dosage for an ion implantation process utilizing boron as the dopant. Data was obtained by a TP-400 XP system after an ion implantation process was conducted on a silicon wafer by using boron as dopant at an implantation energy of 100 KeV. As shown in FIG. 2, the higher the implantation dosage used, the larger the damage made to the silicon crystal. This is indicated by the increased thermal wave signal. The thermal wave measurement conducted on an implanted silicon wafer surface therefore provides a convenient method for indicating the damage inflicted in a crystal structure after an ion implantation process is conducted on the silicon wafer.
A problem incurred in making thermal wave measurements on an implanted silicon surface is the instability of the data obtained immediately after the wafer is implanted. The phenomenon is attributed to the fact that after ions are implanted into a silicon substrate, even though the crystal lattice in the surface layer is damaged by the implanted ions, the displaced atoms in the crystal lattice have the tendency and capability of self-repairing and thus attempt to repair the damaged structure and to return to its original undamaged state. The self-repairing process is reflected in thermal wave measurements made by the TP-400 XP system by the ever-changing thermal wave. As a result, the degree of damage to the crystal lattice cannot be determined reliably by the thermal wave data. Consequently, the processing parameters for the implantation process cannot be reliably determined and that a fabrication process for volume production of the semiconductor device from a silicon wafer can not be carried out.
It is therefore an object of the present invention to provide a method for stabilizing the crystal structure in a silicon substrate after an ion implantation process that does not have the drawbacks or shortcomings of conventional methods.
It is another object of the present invention to provide a method for stabilizing the crystal structure of a silicon substrate after an ion implantation process by exposing the substrate to a heat treatment temperature that is sufficiently low such that a complete recovery of the crystal structure can not take place.
It is a further object of the present invention to provide a method for stabilizing the crystal structure of a silicon substrate after an ion implantation process by exposing the substrate to a heat treatment temperature not higher than 200.degree. C. such that the damaged crystal structure does not completely recover.
It is still another object of the present invention to provide a method for stabilizing the crystal structure of a silicon substrate after an ion implantation process by exposing the substrate to a heat treatment temperature that is sufficiently low such that only a self-repairing process of the damaged crystal lattice can take place to stabilize the structure.
It is yet another object of the present invention to provide a method for stabilizing the crystal structure of a silicon substrate after an ion implantation process wherein the substrate is exposed to a heat treatment temperature not higher than 200.degree. C. for a time period not less than 10 seconds.
It is another further object of the present invention to provide a method for stabilizing the crystal structure of a silicon substrate after an ion implantation process wherein the substrate is exposed to a temperature that is sufficiently low such that the self-repairing process of the crystal lattice can be saturated while the lattice remains in an implanted state.